A magnetostrictive sensor is typically used for detecting a torque or a load. In many magnetostrictive sensors, a magnetostrictive portion containing a magnetostrictive material is formed on an outer peripheral surface of a member (substrate) on which a torque or a load acts. In a magnetostrictive sensor having such a structure, a coil is disposed to surround the substrate on which the magnetostrictive portion is formed.
The above-described structure of the magnetostrictive sensor causes a magnetic permeability of the magnetostrictive material of the magnetostrictive portion to change when a force acts on the substrate on which the magnetostrictive portion is formed. Accordingly, the impedance of the coil disposed to surround the magnetostrictive portion changes. The magnetostrictive sensor detects the change in the impedance of the coil, thereby detecting a torque or a load.
Sensitivity of such a magnetostrictive sensor is affected by a magnetostriction constant and a magnetic permeability of a magnetostrictive material. The magnetostriction constant and the magnetic permeability of the magnetostrictive material vary depending on a chemical composition of the magnetostrictive material. For example, as disclosed in Non-patent Document 1, a Fe—Ni alloy shows significant changes of the magnetostriction constant and the magnetic permeability depending on the Ni content in the alloy. As also disclosed in Non-patent Document 2, for example, a Fe—Co alloy shows significant changes of the magnetostriction constant and the magnetic permeability depending on the Co content in the alloy.
In view of this, in techniques employed to date, the chemical composition of the magnetostrictive material for use in a magnetostrictive sensor is adjusted to increase sensitivity of the magnetostrictive sensor. Specifically, the chemical composition of the magnetostrictive material is adjusted in such a manner that the magnetostriction constant and the magnetic permeability have values suitable for increasing sensitivity of the magnetostrictive sensor.
For example, Patent Document 1 discloses a magnetostrictive torque sensor including a magnetostrictive film made of a Ni—Fe alloy. In the magnetostrictive torque sensor of Patent Document 1, the chemical composition of the Ni—Fe alloy (magnetostrictive film) is adjusted in such a manner that the product of the magnetostriction constant and the magnetic permeability is 90% or more of the maximum value thereof.